KIR2DL4 (2DL4; CD158d) is structurally and functionally unique among members of the killer cell Ig-like receptor (KIR) family in humans. Studies indicate that 2DL4 is the only KIR for which mRNA is expressed in all NK cell clones analyzed, which strongly indicates that it serves a biologically important function. It is also the only KIR that reportedly binds HLA-G, which is almost exclusively expressed on fetal-derived trophoblasts that infiltrate the maternal decidua in pregnant women. Thus, 2DL4 has been proposed to play an important role during pregnancy, although additional functional roles are clearly possible, particularly in cancer and virus infection. 2DL4 also stands out as the only NK cell activating receptor that reportedly triggers interferon (IFN)gamma production, but not target cell cytotoxicity in resting human NK cells. In contrast, other NK cell activating receptors initiate a functional response program that leads to both IFNgamma production and cytotoxicity. We hypothesize that the distinctive functional attributes of 2DL4 result from physical linkage to a unique transmembrane accessory protein that couples the receptor to signal transduction cascades that differ from those triggered by other activating receptors. Our preliminary data support this hypothesis. We propose to define the molecular basis for the distinctive functional response program activated by 2DL4 and define the elements that differ from those triggered by other NK cell activating receptors that also stimulate cytolytic responses. The results will allow us to define the molecular basis for activating distinct functional response programs in NK cells when they encounter trophoblasts, tumor cells, or virus infected cells. We will pursue the following specific aims to achieve this goal: 1 How do structural elements of KIR2DL4 contribute to its unique functions? 2. What accessory signaling protein is associated with the transmembrane domain of KIR2DL4 to transduce intracellular signals? 3. What are the molecular mechanisms by which KIR2DL4 transduces unique activation signals inhuman NK cells?